This research is directed toward understanding the two primary functions of low density lipoprotein (LDL), i.e., the transport of cholesterol and phospholipid and the transfer of these lipids to the cell, which involves the binding of LDL to its cell membrane receptor. To understand these processes, we must determine the structure of the apoprotein of LDL (apo B). Recently we have demonstrated that the cellular receptor binding specificity of LDL is a property of apo B. We have also shown that by selective trypsinization of native LDL one may remove approximately 30% of the protein and yet there remains an intact lipoprotein containing a family of polypeptides of molecular weights 14,000-100,000. These tryptic polypeptides retain the lipid-binding properties of apo B, and following delipidation they also bind to the cellular LDL receptor. Our research objectives are initially to separate these polypeptides and then to determine the ability of individual peptides to bind to the cell receptor and to bind lipid. The peptide separations are by chromatographic methods using denaturants to reduce peptide aggregation. Screening of peptides to determine their ability to bind to the LDL receptor will be by demonstrating inhibition of 125I-LDL binding. In studies of lipid binding to apo B, a variety of physical methods will be utilized as well as an immunological approach using anti-native LDL gamma G antibodies. These antibodies will initially be fractionated using the isolated apo B tryptic peptides. Such subpopulations of antibodies should recognize determinants on apo B as these exist in a native conformation stabilized in the presence of lipid, and thus they can be used as probes to measure the reconstitution with lipid of apo B or of the tryptic polypeptides. These immunologic studies will also enable us to begin defining the structure and orientation of the antigenic determinants of LDL. Ultimately we hope to relate the biological properties of LDL to the structure of apolipoprotein B.